feat: add favicon.ico and gate model write APIs by env flag

- Generate and serve real /favicon.ico from static assets
- Update HTML to use /favicon.ico
- Add ENABLE_MODEL_RW_API setting (default: false)
- Disable POST/DELETE model endpoints by default
- Hide write endpoints from OpenAPI when disabled
- Return 404 for write endpoints when disabled
- Update env.example with ENABLE_MODEL_RW_API documentation
- Update README and PRD with R/W API policy and remote compose notes
- Add tests to verify write endpoints are disabled by default

main.py

@@ -61,6 +61,12 @@ async def dashboard():
     """Dashboard principale"""
     return FileResponse(templates_path / "index.html")
 
+
+@app.get("/favicon.ico", include_in_schema=False)
+async def favicon():
+    """Favicon dell'applicazione."""
+    return FileResponse(static_path / "favicon.ico")
+
 # Event hooks
 @app.on_event("startup")
 async def startup_event():